1. Field of the Invention
The present invention relates to a device for measuring the length of linear movement in which a displacement in the linear direction is converted into a displacement in the rotational direction thereby detecting the displacement in the rotational direction by means of a rotary potentiometer and outputting as electric signals.
2. Description of the Related Art
The measuring device of this type is suitable as a simple displacement detector to detect displacements of a back-and-forth movement of a sheet, a footrest, a backrest, etc. for aircrafts for properly controlling a position or an angle of seats. Besides the aircrafts, it is also useful as a general device for measuring the displacement in the linear movement by detecting the displacement in the electric signal with relatively easy operations.
The measuring device described above may include a means using a wire and a drum for converting the linear displacement into the rotational displacement.
The measuring device of this type is constructed in such a manner that an end of the wire is connected to the drum combined with an input shaft of the rotary potentiometer, and the other end (free end) of the wire is connected to the object to be measured. With this construction, the linear displacement of the object is transferred to the wire so as to rotate the drum and converted into the rotational displacement.
In this arrangement, the rotation of the drum is transmitted to the rotary potentiometer and then electric signals defined by a size according to the rotating angle are taken out from the potentiometer, so that the linear displacement is measured according to the size.
In such a measuring device, a measurement accuracy is not promised unless the displacement of the wire and the rotational angle of the drum achieved by the linear movement of the object to be measured are in a proportional relationship (basically in a direct proportion).
Accordingly, a regular winding mechanism is used for regularly winding the wire in the axial direction of a drum surface by making a single layer when winding the wire in such a manner that both winding of the wire onto the drum surface and unwinding the same from the drum surface are always performed to have a equivalent radius from the center of rotation of the drum.
In conventional measuring devices, the regular winding mechanism is constructed as that a wire guide is moved by the distance of a wire diameter synchronizing with one revolution of the drum while the wire guide is moved parallel in the axial direction of the drum surface.
However, the following problems can be found in the related art.
In order to regularly wind the wire to make a single layer in the axial direction of the drum surface, the wire guide has to be precisely moved in synchronization with the rotation of the drum, so that the construction of the regular winding mechanism became complicated and sizable giving a difficulty to make the construction miniaturized.
For achieving a precise measurement constantly without disarranging the regular winding when winding or unwinding the wire, it is preferable for the wire to be wound or unwound in the direction approximately orthogonal to the axial direction of the drum surface, but this point has not been considered.
Similarly, it is very effective for the constant precise measurement to realize that both winding and unwinding of the wire are always performed at the same position, in other words, the drum is moved in the axial direction without moving the wire for the regular winding. However, this point is also not considered.
In addition to the above, since the wire needs to be kept in a tension all the time for achieving a precise measurement, it becomes unavoidable for the construction to be sizable.
In the light of the above, it is an object of the present invention to provide a device for measuring the length of the linear movement in which a precise measurement can be constantly secured and the construction thereof can be easily miniaturized.
For the object described above, the first aspect of the present invention comprises: a rotary potentiometer for outputting electric signals according to the rotational angle of an input shaft; a housing fixedly mounted integrally with the potentiometer and formed so as to surround the input shaft of the potentiometer; a drum stored in the housing and having a cylindrical drum surface that rotates coaxially and integrally with the input shaft and being movable in the axial direction of the input shaft; a linear body like a wire the rear end of which is connected to one axial end of the drum and the front end of which is regularly wound toward the other axial end of the drum surface forming a single layer and extends in the direction almost orthogonal to the axis of the drum surface through the housing toward the outside thereof to be connected to the object to be measured; a resilient body like a spiral spring for applying deflection forces on the drum in the direction to wind the linear body on the drum surface; and a screw feeding mechanism setting screw pitches so as to axially move the drum by the distance of circumference-length corresponding to one pitch of the linear body regularly wound in every revolution of the drum.
In the second aspect of the present invention, the resilient body is a spiral spring formed concentrically with the drum.
In the third aspect of the present invention, the portion of the housing where the linear body passes through is mounted with a linear body guide made of fluoroplastic, and the linear body is guided through a linear body insertion hole formed on the guide toward the exterior of the housing.
In the forth aspect of the present invention, the linear body is a stranded wire or like a cable made of stainless steel.
In the first aspect of the present invention, the linear body and the drum convert a linear displacement of the object to be measured into a rotational displacement enabling to measure the linear displacement of the object by the rotary potentiometer.
The screw feeding mechanism moves the drum in the axial direction when rotating the drum. Here, the screw feeding mechanism moves the drum in the axial direction by the distance approximately corresponding to a pitch of the linear body regularly wound in one revolution of the drum so that both winding and unwinding of the linear body are always performed at the same position of the housing.
In this arrangement, the linear body is regularly wound appropriately forming a single layer in the axial direction of the drum surface when being wound, and winding the linear body on the drum surface and unwinding the same from the drum surface are always performed to have an equivalent radius from the center of the rotation of the drum.
Therefore, a proportional relationship can be established among the length of the displacement of the body to be measured, the displacement angle of the drum, and the rotational angle of the input shaft of the rotary potentiometer, thereby increasing a measuring accuracy of the linear displacement of the object by the output signals of the potentiometer.
The resilient body applies back tension forces to the drum in the direction in which the linear body is wound on the drum surface so that an accurate measurement can be secured in a state where the linear body is strained.
In the second aspect of the present invention, the resilient body is constructed of a spiral spring concentrically formed with the drum so that the axial dimension can be reduced compared with other resilient methods.
In the third aspect of the present invention, the linear body is guided exterior of the housing through the linear body insertion hole on the linear body guide made of fluoroplastic. The linear body guide acts to reduce contact friction and abrasion caused by a contact with the linear body when winding and unwinding the linear body.
In the forth aspect of the present invention, the linear body is formed of a stranded wire made of stainless steel. The stainless steel works as a rust prevention, and the stranded wire prevents loosening and extending in the course of time after a long-period employment.